Magnetic shimming is the process of reducing magnetic field inhomogeneities. It is an important aspect of optimizing image quality 1.
Field inhomogeneities can result from both intrinsic magnet properties and the surrounding environment of the MR magnet - such as the presence of nearby metallic and electrical structures and the fields of nearby scanners. Magnetic field homogeneity must be considered patient-by-patient to fine-tune the resulting image resolution and quality 1.
There are two main types of shimming: passive and active (sometimes both may be used) 1:
active shimming uses shim coils, which have electric currents directed through them to generate their own magnetic field canceling the remaining field gradients in the main magnetic field
passive shimming uses ferromagnetic pellets or pieces of sheet metal attached to the MRI scanner bore
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